The present invention relates to an improvement of bonding of a semiconductor chip to a die pad of a lead frame.
Conventionally, a semiconductor device is produced by first bonding a semiconductor chip to a die pad of a lead frame (what is called the die bonding), performing electrical connections such as wire bonding to the respective leads, and finally sealing this structure by forming a resin (hereinafter called molding). As shown in FIG. 6, in the die bonding of a semiconductor chip 4, a preform material 10 that is a bonding material such as Au, Au--Si, a solder or a paste is applied to a die pad 8 of a lead frame that is a metal thin plate made of, e.g., a 42--Ni alloy.
Where the preform material 10 is a metallic material, it needs to be heated to a high temperature in the bonding process. For example, in the case of the Au--Si preform material, the die pad 8 is pre-heated at about 300.degree. C. for several seconds, further heated to 500.degree.-600.degree. C. at the time of the bonding, and then gradually cooled.
In the case of metallic preform materials, such as Au, Au--Si and a solder, which need to be heated to a high temperature in the bonding process, a thermal stress is exerted on the semiconductor chip to affect the semiconductor characteristics. Further, several heating stages, for instance, stages for pre-heating at 300.degree. C., bonding at 500-600.degree. C., gradual cooling at 300.degree. C., 100.degree. C. and 25.degree. C. (ordinary temperature), should be prepared, which causes an increase of costs of manufacturing equipment.
Since the paste preform material is made, for instance, of an epoxy resin that is made conductive by adding a silver powder etc., the bonding process itself can be performed at the ordinary temperature. However, to set the resin, a large number of bonded structures are collected and subjected to aging in an oven at about 170.degree. C. for 10-20 hours.
However, the existence of the aging step lowers the efficiency of the bonding process and causes a cost increase. Further, there exist some factors of deteriorating the semiconductor characteristics: for instance, the semiconductor device is placed in an environment of a relatively high temperature of about 170.degree. C., and a volatile organic gas generated from the paste may corrode the solder.
In addition, since MOS semiconductor devices need very little current, it is becoming less important to dissipate heat through the semiconductor substrate and the die pad, and to make a current flow through the back face of the semiconductor substrate. That is, it is now desired that adverse influences on the semiconductor characteristics be prevented and the efficiency of the die bonding process be improved, rather than considering such properties as electrical conduction and thermal conduction of the die bonding material.